The four-show crossover got off to a great start on last night's episode of The Flash (Barry and Cisco only made a brief cameo on Monday night's Supergirl). But tonight's Arrow is undoubtedly the most heavily anticipated segment of the four-part story, as it marks the 100th episode of the show to air on the CW. Featuring a cast filled with guest stars from the show's past, "Invasion!" proves to be one of Arrow's most memorable episodes of all time.

Having just been beamed up into the Dominators' spacecraft, Ollie, Sara, Ray, Thea, and Diggle are put in special stasis chambers where they undergo a collective hallucination induced by the Dominators. As the episode is essentially a loose adaptation of Alan Moore's renowned Superman story, "For the Man Who Has Everything," the heroes inhabit a dream world where everything they could ever want is given to them. Ollie is marrying Laurel, who is still alive along with Ollie's parents. Diggle is the Green Arrow, and Tommy Merlyn never died in the Glades. However, over the course of the episode, each of the heroes has flashbacks of their true lives and start to catch on that what they're seeing isn't real.

When the team finally makes the decision to leave the dream world behind, they learn that the Dominators have put "safeguards" in place to ensure they don't escape. Manifestations of Merlyn, Slade Wilson, Andy Diggle, Damien Darhk, and others appear to fight the heroes, but of course, Team Arrow emerges victorious. They then head for a building in town that seems out of place, and there they find a portal back to reality. Once they're back on the other side, they jump in an escape pod and are picked up by the Wave Rider. Though all the heroes are now back in commission, the episode ends as they learn the aliens have a secret weapon and are heading right for Earth.

Arrow's 100th episode definitely delivered on many levels. Seeing members of the old cast back on the show is truly a nostalgic experience, and the episode lives up to the same emotional heights that Moore reached in the comic that inspired it. The choreography and action sequences are also absolutely on point for "Invasion!," with the mega-brawl at the end and Kara and Barry's attempts to take out a cyborg enemy being the highlights. Just as well, Mr. Terrific's disappointment with mankind's first extraterrestrial contact serves as effective comic relief, interjecting an otherwise extremely emotional experience with a bit of levity.

There are a few things that could be better, however. Certain parts of the plot seem somewhat unbelievable, as there's no real explanation for why Ollie and the team are just able to walk out of their hallucination. Likewise, why the aliens would only program the reality with a handful of safeguards instead of myriads of them is never answered, and it makes things seem just a little too convenient. Just as well, while adapting "For the Man Who Has Everything" genuinely worked for the episode, Berlanti just adapted it last year for Supergirl.

Overall, however, Arrow's 100th episode is almost everything that DC fans could have wanted. The acting, the action, the emotion - tonight's Arrow delivers on each. The show hasn't been this strong in a very long time, and the cast and crew certainly deserve heartfelt congratulations for hitting such a big milestone and inspiring the entire DC TV universe.

Heroes vs. Aliens continues tomorrow for the final part of the story on Legends of Tomorrow at 8 p.m. ET.